The present invention relates, in general, to a magnetic field guidance system and, more particularly, to a guidance system utilizing magnetic field sources for use in guiding a movable carrier, such as a continuously steerable borehole drilling system, with respect to a target magnetic field sensor.
Guidance systems utilizing varying magnetic fields superimposed on the earth's magnetic field have long been known and have been used for a wide variety of purposes. A principle use of such systems has been found in the guidance of steerable drills for the drilling of horizontal boreholes, for example, in the manner illustrated and described in U.S. Pat. Nos. 3,731,752 issued to Charles A. Schad, 3,406,766 to John K. Henderson, and 4,646,277 to Jack E. Bridges et al, but such systems are also useful in other arenas as diverse as, for example, well logging, instrument landing systems for aircraft, navigation systems for ships, and the like.
Patents such as those of Schad, Henderson and Bridges operate by producing a magnetic field at one location, sensing that field at a second location, and controlling a movable carrier such as a drill in accordance with the sensed field. In general, systems of the type represented by the Schad, Henderson and Bridges work well to direct a drill toward a target location, but have the disadvantage of not providing information about the relative distance or the direction of the target. Furthermore, some of these systems may cause the drill to follow magnetic field lines, rather than traveling toward the target.
Other patents, such as that shown in U.S. Pat. No. 4,812,812 to Flowerdew attempt to overcome these problems, for example, by providing acceleratometers to locate the vertical and to provide a reference for a rotating magnetic field. However, such systems require the use of additional instrumentation, and may also be prone to error if the measuring instrument is either moving or vibrating.
Still other systems for guiding horizontal drilling, such as that disclosed in U.S. Pat. Nos. 3,529,682 and 3,589,454 to James C. Coyne, have relied upon surface wires to provide magnetic fields which will guide underground drills. However, the use of such surface wires is often inconvenient, and may not be possible where there are intervening property owners or intervening surface obstacles, or where the drilling to be guided is at a great depth.